Non-alkali glass, and process for production of non-alkali glass

ABSTRACT

The present invention relates to an alkali-free glass having a strain point of 735° C. or higher, an average thermal expansion coefficient at from 50 to 350° C. of from 30×10 −7  to 40×10 −7 /° C., a temperature T 2  at which a glass viscosity is 10 2  dPa·s of 1,710° C. or lower, a temperature T 4  at which a glass viscosity is 10 4  dPa·s of 1,340° C. or lower, and a devitrification temperature of 1,330° C. or lower, the alkali-free glass including, in terms of mol % on the basis of oxides: SiO 2  66 to 69, Al 2 O 3  12 to 15, B 2 O 3  0 to 1.5, MgO 6 to 9.5, CaO 7 to 9, SrO 0.5 to 3, BaO 0 to 1, and ZrO 2  0 to 2, in which MgO+CaO+SrO+BaO is from 16 to 18.2, MgO/(MgO+CaO+SrO+BaO) is 0.35 or more, MgO/(MgO+CaO) is 0.40 or more and less than 0.52, and MgO/(MgO+SrO) is 0.45 or more.

TECHNICAL FIELD

The present invention relates to an alkali-free glass that is suitable as various substrate glasses for a display and substrate glasses for a photomask, does not substantially contain an alkali metal oxide and can be subjected to float forming.

BACKGROUND OF THE INVENTION

The following characteristics have conventionally been required in various substrate glasses for a display, particularly substrate glasses having a metal or an oxide thin film formed on the surface thereof.

(1) In the case where an alkali metal oxide is contained, alkali metal ions diffuse in the thin film, resulting in deterioration of film characteristics. For this reason, alkali metal ions are not substantially contained. (2) A strain point is high such that deformation of a glass and shrinkage (thermal shrinkage) due to structure stabilization of a glass can be minimized when exposed to high temperature in a thin film formation step. (3) A glass has sufficient chemical durability to various chemicals used in semiconductor formation. Particularly, the glass has durability to buffered hydrofluoric acid (BHF: mixed liquid of hydrofluoric acid and ammonium fluoride) for etching SiO_(x), or SiN_(x)), a chemical solution containing hydrochloric acid used for etching ITO, various acids (nitric acid, sulfuric acid and the like) used for etching an metal electrode, and an alkaline of a resist stripping solution. (4) Defects (bubble, striae, inclusion, pit, flaw and the like) are not present in the inside and on the surface.

In addition to the above requirements, the recent years are under following situations.

(5) Reduction in weight of a display is required, and a glass itself is required to have small density. (6) Reduction in weight of a display is required, and decrease in thickness of a substrate glass is desired. (7) In addition to the conventional amorphous silicon (a-Si) type liquid crystal display, a polycrystal silicon (p-Si) type liquid crystal display in which heat treatment temperature is slightly high has began to be produced (a-Si: about 350° C.→p-Si: 350 to 550° C.). (8) A glass having small average thermal expansion coefficient is required in order to improve productivity and increasing thermal shock resistance by increasing a temperature-rising rate in a heat treatment for liquid display preparation.

On the other hand, dry etching proceeds and requirement to BHF resistance is becoming to weaken. Many glasses conventionally used are glasses containing 6 to 10 mol % of B₂O₃ in order to improve BHF resistance. However, B₂O₃ has the tendency to decrease a strain point. The following glasses are exemplified as an alkali-free glass that does not contain B₂O₃ or contains B₂O₃ in small amount.

Patent Document 1 discloses a glass containing 0 to 5% by weight of B₂O₃, Patent Document 2 discloses a glass containing 0 to 5 mol % of B₂O₃, and Patent Document 3 discloses a glass containing 0 to 8 mol % of B₂O₃.

BACKGROUND ART Patent Documents

-   Patent Document 1: JP-A-63-176332 -   Patent Document 2: JP-A-5-232458 -   Patent Document 3: JP-A-8-109037 -   Patent Document 4: JP-A-10-45422

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

However, the glass described in Patent Document 1 contains 11 mol % or more CaO. Therefore, the glass has high devitrification temperature, contains large amount of impurity phosphorus present in limestone that is a raw material of CaO, and may cause leak current in a transistor prepared on a glass substrate.

Moreover, the glass described in Patent Document 2 contains 15 mol % or more of SrO. Therefore, an average thermal expansion coefficient at from 50 to 300° C. exceeds 50×10⁻⁷/° C.

Moreover, the glass described in Patent Document 3 is classified into “a glass containing 55 to 67% by weight of SiO₂ and containing 6 to 14% by weight of Al₂O₃” (group “a”) and “a glass containing 49 to 58% by weight of SiO₂ and containing 16 to 23% by weight of Al₂O₃” (group “b”). The group “a” contains a large amount of SiO₂, and therefore has the problem that a silica sand that is SiO₂ raw material does not completely melt in a melt and remains as an unmated silica sand. The group “b” contains a large amount of Al₂O₃, and therefore has the problem that a devitrification temperature is remarkably increased.

To solve the problems in the glasses described in Patent Documents 1 to 3, an alkali-free glass described in Patent Document 4 is proposed. The alkali-free glass described in Patent Document 4 has high strain point, can be formed by a float process, and is considered to be suitable for use in a display substrate, a photomask substrate and the like.

A production method of high quality p-Si TFT includes solid phase crystallization method. However, to carry out the method, it is required to further increase a strain point.

Furthermore, to comply with the demand in glass production process, particularly melting and forming, a glass is required to have lower viscosity and low devitrification property.

An object of the present invention is to provide an alkali-free glass that solves the above disadvantages, has high strain point, low viscosity and low devitrification property, and is easily subjected to float forming.

Means for Solving the Problems

The present invention provides an alkali-free glass having a strain point of 735° C. or higher, an average thermal expansion coefficient at from 50 to 350° C. of from 30×10⁻⁷ to 40×10⁻⁷/° C., a temperature T₂ at which a glass viscosity is 10² dPa·s of 1,710° C. or lower, a temperature T₄ at which a glass viscosity is 10⁴ dPa·s of 1,340° C. or lower, and a devitrification temperature of 1,330° C. or lower, the alkali-free glass comprising, in terms of mol % on the basis of oxides:

SiO₂  66 to 69, Al₂O₃  12 to 15, B₂O₃   0 to 1.5, MgO   6 to 9.5, CaO   7 to 9, SrO 0.5 to 3, BaO   0 to 1, and ZrO₂   0 to 2,

wherein MgO+CaO+SrO+BaO is from 16 to 18.2,

MgO/(MgO+CaO+SrO+BaO) is 0.35 or more,

MgO/(MgO+CaO) is 0.40 or more and less than 0.52, and

MgO/(MgO+SrO) is 0.45 or more.

Advantage of the Invention

The alkali-free glass of the present invention is a glass that is suitable for a display substrate, a photomask substrate and the like that are for uses at high strain point, and is easily subjected to float forming.

Mode for Carrying Out the Invention

Composition range of each component is descried below. In the case where SiO₂ is less than 66% (mol %; unless otherwise indicated, thereinafter the same), a strain point is not sufficiently increased, a thermal expansion coefficient is increased and density is increased. The content of SiO₂ is preferably 67% or more. However, in the case where the content exceeds 69%, melting performance is decreased and a devitrification temperature is increased.

Al₂O₃ inhibits phase separation property of a glass, decreases a thermal expansion coefficient and increases a strain point. However, in the case where the content of Al₂O₃ is less than 12%, the effect is not achieved, and other components increasing expansion are increased. As a result, thermal expansion becomes large. The content of Al₂O₃ is preferably 13.5% or more. However, in the case where the content exceeds 15%, melting performance of a glass is deteriorated, and a devitrification temperature may be increased. The content is preferably 14.5% or less.

Since B₂O₃ improves melting reaction property of a glass, and decreases a devitrification temperature, B₂O₃ can be added up to 1.5%. However, in the case where the content is too large, a strain point is decreased. Therefore, the content is preferably 1% or less. Considering environmental load, it is preferred that B₂O₃ is not substantially contained (that is, B₂O₃ is not contained except that B₂O₃ is unavoidably contained as impurities; hereinafter the same).

MgO has the characteristics that it does not increase expansion and do not excessively decrease a strain point, among alkali earths. MgO also improves melting performance, but in the case where the content thereof is less than 6%, the effect is not sufficiently achieved. The content is preferably 7% or more. However, in the case where the content exceeds 9.5%, a devitrification temperature may be increased. The content is preferably 8.5% or less.

CaO has the characteristics that it does not increase expansion next to MgO among alkali earths, and do not excessively decrease a strain point than MgO, and additionally improves melting performance. However, in the case where the content is less than 7%, the effect is not sufficiently achieved. The content is preferably 7.5% or more. However, in the case where the content exceeds 9%, a devitrification temperature may be increased and a phosphorus that is impurities in lime (CaCO₃) that is CaO raw material may be incorporated in large amount. The content is preferably 8.5% or less.

SrO improves melting performance without increasing a devitrification temperature of a glass. In the case where the content is less than 0.5%, the effect is not sufficiently achieved. The content is preferably 1% or more. However, SrO has the tendency to increase a expansion coefficient as compared with MgO and CaO, and in the case where the content exceeds 3%, a thermal expansion coefficient may be increased.

BaO is not essential, but can be contained to improve melting performance. However, BaO has the tendency to increase a thermal expansion coefficient as compared with MgO and CaO, and in the case where the content is too large, expansion and density of a glass are excessively increased. Therefore, the content is 1% or less. It is preferred that BaO is not substantially contained.

ZrO₂ may be contained up to 2% in order to decrease a glass melting temperature. In the case where the content exceeds 2%, a glass becomes unstable, or a dielectric constant ∈ of a glass is increased. The content is preferably 1.5% or less, and it is more preferred that ZrO₂ is not substantially contained.

In the case where the total content of MgO, CaO, SrO and BaO is less than 16%, melting performance becomes poor. The total content is preferably 17% or more. However, in the case where the total content is more than 18.2%, there may be a difficulty that a thermal expansion coefficient cannot be decreased. The total content is preferably 18% or less.

When the following three requirements are satisfied, a strain point can be increased without increasing a devitrification temperature, and additionally, a viscosity of a glass can be decreased.

MgO/(MgO+CaO+SrO+BaO) is 0.35 or more, and preferably 0.37 or more.

MgO/(MgO+CaO) is 0.40 or more and less than 0.52, and preferably 0.45 or more and less than 0.52.

MgO/(MgO+SrO) is 0.45 or more, and preferably 0.5 or more.

Incidentally, the alkali-free glass of the present invention does not contain an alkali metal oxide in an amount exceeding impurity level (that is, does not substantially contain) in order to avoid the occurrence of deterioration in characteristics of a metal or a thin oxide film, provided on a glass surface during panel production. Moreover, for the same reason, it is preferred that P₂O₅ is not substantially contained. Furthermore, to facilitate recycle of a glass, it is preferred that PbO, As₂O₃ and Sb₂O₃ are not substantially contained.

In addition of the above components, to improve melting performance, refining and float formability of a glass, ZnO, Fe₂O₃, SO₃, F, Cl and SnO₂ can be added in the total content of 5% or less to the alkali-free glass of the present invention.

The alkali-free glass of the present invention has a strain point of 735° C. or higher, preferably 737° C. or higher and more preferably 740° C. or higher, and therefore a thermal shrinkage during panel production can be suppressed. Moreover, solid phase crystallization method can be applied as a production method of p-Si TFT.

Incidentally, the alkali-free glass of the present invention has a strain point of 735° C. or higher, and is therefore suitable for uses at high strain point (for example, a display substrate or a lighting substrate for an organic EL, or a thin display substrate or a lighting substrate, having a thickness of 100 μm or less).

Forming of a sheet glass having a thickness of 100 μm or less has the tendency that drawing rate during forming becomes fast. As a result, a fictive temperature of a glass is increased, and compaction of a glass is increased easily. In this case, when a glass is a high strain point glass, compaction can be inhibited.

Moreover, the alkali-free glass of the present invention has a glass transition point of preferably 760° C. or higher, more preferably 770° C. or higher, and still more preferably 780° C. or higher.

Also, the alkali-free glass of the present invention has an average thermal expansion coefficient at 50 to 350° C. of from 30×10⁻⁷ to 40×10⁻⁷/° C., has large thermal shock resistance and can increase productivity when producing a panel. It is preferred in the alkali-free glass of the present invention that the average thermal expansion coefficient at 50 to 350° C. is from 35×10⁻⁷ to 40×10⁻⁷/° C.

Furthermore, the alkali-free glass of the present invention has a specific gravity of preferably 2.65 or less, more preferably 2.64 or less, and still more preferably 2.62 or less.

Moreover, according to the alkali-free glass of the present invention, a temperature T₂ at which a viscosity η is 10² poise (dPa·s) is 1,710° C. or lower, preferably 1,700° C. or lower and more preferably 1,690° C. or lower. Therefore, melting of the glass is relatively easy.

Furthermore, according to the alkali-free glass of the present invention, a temperature T₄ at which a viscosity η is 10⁴ poise is 1,340° C. or lower, preferably 1,335° C. or lower, and more preferably 1,330° C. or lower. Therefore, the glass is suitable for float forming.

Moreover, the alkali-free glass of the present invention has a devitrification temperature of 1,330° C. or lower, preferably lower than 1,300° C., and more preferably 1,290° C. or lower. Therefore, forming by a float process is easy.

The devitrification temperature in the present description is an average value of a maximum temperature at which crystals precipitate on the surface of a glass and in the inside thereof, and a minimum temperature at which crystals do not precipitate, by placing crushed glass particles on a platinum dish, conducting heat treatment for 17 hours in an electric furnace controlled to constant temperature, and observing with an optical microscope after the heat treatment.

Considering devitrification prevention during forming a sheet glass, it is preferred to satisfy a temperature T₄−a devitrification temperature of the glass≧0° C., and furthermore a temperature T₄−a devitrification temperature of the glass≧20° C., in the case of a float process.

Moreover, the alkali-free glass of the present invention preferably has Young's modulus of 84 GPa or more, furthermore 86 GPa or more, furthermore 88 GPa or more, and furthermore 90 GPa or more.

Moreover, the alkali-free glass of the present invention preferably has a photoelastic constant of 31 nm/MPa/cm or less.

When a glass substrate has a birefringence by stress generated in a liquid crystal display panel production step and during using a liquid crystal display, display of black becomes gray, and the phenomenon that contrast of a liquid crystal display is decreased is sometimes recognized. When the photoelastic constant is 31 nm/MPa/cm or less, this phenomenon can be inhibited small. The photoelastic constant is preferably 30 nm/MPa/cm or less, more preferably 29 nm/MPa/cm or less, still more preferably 28.5 nm/MPa/cm or less, and particularly preferably 28 nm/MPa/cm or less. Also, considering easiness of securing other properties, the photoelastic constant is preferably 25 nm/MPa/cm or less.

Incidentally, the photoelastic constant can be measured by a disk compression method.

The alkali-free glass of the present invention preferably has a dielectric constant of 5.6 or more.

In the case of In-Cell type touch panel (touch sensor is incorporated in a liquid crystal display panel) as described in JP-A-2011-70092, it is better that the glass substrate has higher dielectric constant from the standpoints of improvement in sensing sensitivity of a touch sensor, decrease in drive voltage and electric power saving. When the dielectric constant is 5.6 or more, sensing sensitivity of a touch sensor is improved. The dielectric constant is preferably 5.8 or more, more preferably 6.0 or more, still more preferably 6.2 or more, and particularly preferably 6.4 or more.

The dielectric constant can be measured according to the method described in JIS C-2141 (1992).

The alkali-free glass of the present invention can be produced by, for example, the following method. Raw materials of each component generally used are mixed so as to obtain a target component, and the resulting mixture is continuously introduced in a melting furnace, and heated at from 1,500 to 1,800° C. to melt the same. The molten glass obtained is formed into a given sheet thickness by a float process, followed by annealing and cutting. Thus, a sheet glass can be obtained.

Since, the alkali-free glass of the present invention has relatively low melting performance, the following materials are preferably used as raw materials of each component.

(Silicon Source)

A silica sand can be used as a silicon source of SiO₂. When a silica sand in which a median diameter D₅₀ is from 20 to 27 μm, the proportion of particles having a particle size of 2 μm or less is 0.3% by volume or less and the proportion of particles having a particle size of 100 μm or more is 2.5% by volume or less is used, the silica sand can be melted while inhibiting the aggregation of the silica sand. This facilitates melting of the silica sand, and an alkali-free glass having less bubbles and having high homogeneity and flatness is obtained, which is preferred.

Incidentally, the “particle size” in the present specification means a sphere equivalent size (primary particle size in the present invention) of a silica sand, and is specifically a particle size in particle size distribution of a powder measured by a laser diffraction/scattering method.

Moreover, the “median diameter D₅₀” in the present specification means a particle size where, in a particle size distribution of a powder measured by a laser diffraction method, volume frequency of particles having a particle size larger than a certain particle size occupies 50% of that of the whole powder. In other words, the “median diameter D₅₀” means a particle size when the cumulative frequency is 50% in particle size distribution of a powder measured by a laser diffraction method.

Also, the “proportion of particles having a particle size of 2 μm or less” and the “proportion of particles having a particle size of 100 μm or more” in the present specification are measured by particle size distribution with, for example, a laser diffraction/scattering method.

It is preferred that the median diameter D₅₀ of a silica sand is 25 μm or less, because melting of the silica sand becomes easier.

Furthermore, it is particularly preferred that the proportion of particles having a particle size of 100 μm or more in a silica sand is 0% because melting of a silica sand becomes easier.

(Alkaline Earth Metal Source)

An alkali earth metal compound can be used as the alkaline earth metal source. Specific examples of the alkaline earth metal compound include carbonates such as MgCO₃, CaCO₃, BaCO₃, SrCO₃ and (Mg, Ca)CO₃ (dolomite); oxides such as MgO, CaO, BaO and SrO; and hydroxides such as Mg(OH)₂, Ca(OH)₂, Ba(OH)₂ and Sr(OH)₂. When a hydroxide of an alkaline earth metal is contained in a part or the whole of the alkaline earth metal source, the amount of unmelted SiO₂ component during melting glass raw materials is decreased, which is preferred. In the case where the amount of unmelted SiO₂ component contained in a silica sand is increased, the unmelted SiO₂ is incorporated in bubbles when the bubbles are generated in a glass melt, and accumulates near the surface layer of the glass melt. This causes difference in compositional ratio of SiO₂ between the surface layer of the glass melt and part other than the surface layer. As a result, homogeneity of a glass is deteriorated, and additionally, flatness is decreased.

The content of the hydroxide in the alkaline earth metal is preferably from 15 to 100 mol % (in terms of MO), more preferably from 30 to 100 mol % (in terms of MO), and still more preferably from 60 to 100 mol % (in terms of MO), of 100 mol % of the alkaline earth metal source (in terms of MO, provided that M is an alkaline earth metal element). When the content falls within the range, the amount of the unmelted SiO₂ during melting glass raw materials is decreased, which is more preferred.

The amount of the unmelted SiO₂ during melting glass raw materials is decreased with increasing molar ratio of the hydroxide in the alkaline earth metal source. Therefore, higher molar ratio of the hydroxide is preferred.

Specifically, a mixture of a hydroxide of an alkaline earth metal and a carbonate, a hydroxide alone of an alkaline earth metal, and the like can be used as the alkaline earth metal source. At least one of MgCO₃, CaCO₃, and (Mg, Ca)(CO₃)₂ (dolomite) is preferably used as the carbonate. Also, at least one of Mg(OH)₂ and Ca(OH)₂ is preferably used as the hydroxide of the alkaline earth metal. Mg(OH)₂ is particularly preferably used.

(Boron Source)

When the alkali-free glass contains B₂O₃, a boron compound can be used as the boron source of B₂O₃. Specific examples of the boron compound include orthoboric acid (H₃BO₃), metaboric acid (HBO₂), tetraboric acid (H₂B₄O₇) and boric anhydride (B₂O₃). In the production of general alkali-free glass, orthoboric acid is used from the standpoints of inexpensive cost and easy availability.

In the present invention, the boron source preferably contains boric anhydride in an amount of from 10 to 100 mass % (in terms of B₂O₃) of 100 mass % (in terms of B₂O₃) of the boron source. When the boric anhydride contained is 10 mass % or more, aggregation of glass materials is inhibited, and improvement effect in reduction of bubbles, homogeneity and flatness is obtained. The amount of the boric anhydride is more preferably from 20 to 100 mass %, and still more preferably from 40 to 100 mass %.

As a boron compound other than boric anhydride, orthobaric acid is preferred from the standpoints of inexpensive cost and easy availability.

EXAMPLES

Raw materials of each component were mixed so as to obtain a target composition, and melted at a temperature of from 1,500 to 1,600° C. using a platinum crucible. In melting, stirring is conducted using a platinum stirrer to homogenize a glass. The molten glass was flown out, and formed in a plate shape, followed by annealing.

Table 1 shows glass composition (unit: mol %), β OH value of glass (measured by the following procedure as an index of water content in glass, unit: mm⁻¹), thermal expansion coefficient at 50 to 350° C. (unit: ×10⁻⁷/° C.), strain point (unit: ° C.), glass transition point (unit: ° C.), specific gravity, Young's modulus (GPa) (measured by ultrasonic method), temperature T₂ (temperature at which glass viscosity η is 10² poise, unit: ° C.) giving an indication of melting performance and temperature T₄ (temperature at which glass viscosity η is 10⁴ poise, unit: ° C.) giving an indication of float formability, as high temperature viscosity values, devitrification temperature (unit: ° C.), photoelastic constant (unit: nm/MPa/cm) (measured by disk compression method) and dielectric constant (measured according to the method described in JIS C-2141).

[Measurement Method of β OH Value]

Absorbance to light having a wavelength of from 2.75 to 2.95 μm of a glass sample was measured, and the maximum value β_(max) was divided by a thickness (mm) of the sample to obtain β OH value.

Incidentally, in Table 1, values in parentheses are calculated values.

TABLE 1 Example Example Example Example Example Example Example Example Example Mol % 1 2 3 4 5 6 7 8 9 SiO₂ 68.3 68.3 68.5 68.2 68.2 68.3 68.6 68.3 68.3 Al₂O₃ 13.9 13.9 13.5 14.3 14.3 13.5 14.3 13.5 12.9 B₂O₃ 0  0  0  0   0.5 0  0  0  0  MgO  8.1  7.2  8.5 7   7.8  9.1 6   7.5  7.7 CaO  7.8  8.2  8.5  8.5  8.7  8.6 9   8.6  8.8 SrO  1.9  2.4 1  2   0.5  0.5  2.1  1.5  1.7 BaO 0  0  0  0  0  0  0   0.6 0  ZrO₂ 0  0  0  0  0  0  0  0   0.6 Mg + CaO + SrO + BaO 17.8 17.8 18   17.5 17   18.2 17.1 18.2 18.2 Mg/(Mg + CaO + SrO + BaO)  0.46  0.40  0.47  0.40  0.46  0.50  0.35  0.41  0.42 MgO/(MgO + CaO)  0.51  0.47  0.50  0.45  0.47  0.51  0.40  0.47  0.47 MgO/(MgO + SrO)  0.81  0.75  0.89  0.78  0.94  0.95  0.74  0.83  0.82 β OH value  0.3  0.3 Average thermal expansion 37.0 37.6 36.5 36.2 34.4 35.4 36.3 (38.7) (38.0) coefficient (×10⁻⁷/° C.) Strain point (° C.) (742)   (745)   (742)   (746)   (744)   (742)   (749)   (738)   (742)   Glass transition point (° C.) 798   801   807   816   806   808   818   Specific gravity  (2.60)  (2.60)  2.53  2.55  2.52  2.52  2.55  (2.56)  (2.55) Young's modulus (GPa) (87.1) (87.3) 87.0 79.9 84.2 87.0 87.1 (87.0) (87.0) T₂ (° C.) 1674    1677    1683    1685    1678    (1686)    (1695)    (1680)    (1673)    T₄ (° C.) 1328    1327    1322    1326    1320    (1311)    (1349)    (1325)    (1323)    Devitrification temperature 1295    1295    1287    1287    1312    (° C.) Photoelastic constant (29.9) (30.0) 29.7 29.7 29.7 29.7 29.7 (29.5) (29.8) (nm/MPa/cm) Dielectric constant  (6.46)  (6.47)  (6.46)  (6.47)  (6.40)  (6.48)  (6.43)  (6.56)  (6.54)

As is apparent from the table, the glasses of the examples are all that the thermal expansion coefficient is low as 30×10⁻⁷ to 40×10⁻⁷1° C., and the strain point is high as 735° C. or higher. It is seen that the glasses can sufficiently withstand heat treatment at high temperature.

Furthermore, from the fact that the strain point is 735° C. or higher, the glasses are suitable for uses at high strain point (for example, a display substrate or a lighting substrate for an organic EL, or a thin display substrate or a lighting substrate, having a thickness of 100 μm or less).

The temperature T₂ giving an indication of melting performance is relatively low as 1,710° C. or lower, and melting is easy. The temperature T₄ giving an indication of formability is 1,340° C. or lower, and the devitrification temperature is 1,330° C. or lower, and preferably lower than 1,330° C. Therefore, it is considered that the glasses are free from the trouble that devitrification occurs during float forming.

The photoelastic constant is 31 nm/MPa/cm or less, and when the glasses are used as a glass substrate for a liquid crystal display, decrease in contrast can be inhibited.

Furthermore, the dielectric constant is 5.6 or more, and when the glasses are used as a glass substrate for In-Cell type touch panel, sensing sensitivity of a touch sensor is improved.

While the present invention has been described in detail with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Incidentally, the present application is based on Japanese Patent Application No. 2010-289425 filed on Dec. 27, 2010, and the contents are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The alkali-free glass of the present invention has high strain point, can be formed by a float process, and is suitable for use in a display substrate, a photomask substrate, or the like. Furthermore, the glass is also suitable for use in a substrate of solar cell, or the like. 

1. An alkali-free glass having a strain point of 735° C. or higher, an average thermal expansion coefficient at from 50 to 350° C. of from 30×10⁻⁷ to 40×10⁻⁷/° C., a temperature T₂ at which a glass viscosity is 10² dPa·s of 1,710° C. or lower, a temperature T₄ at which a glass viscosity is 10⁴ dPa·s of 1,340° C. or lower, and a devitrification temperature of 1,330° C. or lower, the alkali-free glass comprising, in terms of mol % on the basis of oxides: SiO₂  66 to 69, Al₂O₃  12 to 15, B₂O₃   0 to 1.5, MgO   6 to 9.5, CaO   7 to 9, SrO 0.5 to 3, BaO   0 to 1, and ZrO₂   0 to 2,

wherein MgO+CaO+SrO+BaO is from 16 to 18.2, MgO/(MgO+CaO+SrO+BaO) is 0.35 or more, MgO/(MgO+CaO) is 0.40 or more and less than 0.52, and MgO/(MgO+SrO) is 0.45 or more.
 2. The alkali-free glass according to claim 1, having the strain point of 735° C. or higher, (the temperature T₄−the devitrification temperature of the glass)≧0° C., and Young's modulus of 84 GPa or more, the alkali-free glass comprising, in terms of mol % on the basis of oxides: SiO₂   67 to 69, Al₂O₃ 13.5 to 14.5, B₂O₃   0 to 1, MgO   7 to 8.5, CaO  7.5 to 8.5, SrO   1 to 3, and BaO   0 to 1,

wherein the alkali-free glass does not substantially contain ZrO₂.
 3. A method for producing the alkali-free glass according to claim 1, wherein a silica sand, in which a median diameter D₅₀ is from 20 to 27 μm, a proportion of particles having a particle size of 2 μm or less is 0.3 vol % or less, and a proportion of particles having a particle size of 100 μm or more is 2.5 vol % or less, is used as a silicon source of SiO₂.
 4. A method for producing the alkali-free glass according to claim 1, wherein an alkaline earth metal source containing a hydroxide of an alkaline earth metal in a content of from 15 to 100 mol % (in terms of MO, provided that M is an alkaline earth metal element, and hereinafter the same) of 100 mol % of the alkaline earth metal source is used as the alkaline earth metal source of MgO, CaO, SrO and BaO.
 5. A method for producing the alkali-free glass according to claim 1, wherein a silica sand, in which a median diameter D₅₀ is from 20 to 27 μm, a proportion of particles having a particle size of 2 μm or less is 0.3 vol % or less, and a proportion of particles having a particle size of 100 μm or more is 2.5 vol % or less, is used as a silicon source of SiO₂; and an alkaline earth metal source containing a hydroxide of an alkaline earth metal in a content of from 15 to 100 mol % (in terms of MO, provided that M is an alkaline earth metal element, and hereinafter the same) of 100 mol % of the alkaline earth metal source is used as the alkaline earth metal source of MgO, CaO, SrO and BaO.
 6. A method for producing the alkali-free glass according to claim 2, wherein a silica sand, in which a median diameter D₅₀ is from 20 to 27 μm, a proportion of particles having a particle size of 2 μm or less is 0.3 vol % or less, and a proportion of particles having a particle size of 100 μm or more is 2.5 vol % or less, is used as a silicon source of SiO₂.
 7. A method for producing the alkali-free glass according to claim 2, wherein an alkaline earth metal source containing a hydroxide of an alkaline earth metal in a content of from 15 to 100 mol % (in terms of MO, provided that M is an alkaline earth metal element, and hereinafter the same) of 100 mol % of the alkaline earth metal source is used as the alkaline earth metal source of MgO, CaO, SrO and BaO.
 8. A method for producing the alkali-free glass according to claim 2, wherein a silica sand, in which a median diameter D₅₀ is from 20 to 27 μM, a proportion of particles having a particle size of 2 μm or less is 0.3 vol % or less, and a proportion of particles having a particle size of 100 μm or more is 2.5 vol % or less, is used as a silicon source of SiO₂; and an alkaline earth metal source containing a hydroxide of an alkaline earth metal in a content of from 15 to 100 mol % (in terms of MO, provided that M is an alkaline earth metal element, and hereinafter the same) of 100 mol % of the alkaline earth metal source is used as the alkaline earth metal source of MgO, CaO, SrO and BaO. 